Putting is about half the score in par golf and thus the basic bread and butter of a low score game. The most difficult and elusive element of the stroke is to move the club head on a straight line with the putter face at a right angle to it. The putter must move in a straight line and the face must be kept at a right angle to it throughout the entire stroke--back stroke, fore stroke, at contact with the ball, and during follow through. Small deviations result in magnified inaccuracies in ball travel toward the target.
The golfer must first estimate what he visualizes to be the target line. He then must establish the face of the putter at a right angle to this line. He then must stroke the club against the ball in a straight line along the target line. And he must keep the putter face at a right angle to the line during the entire stroke.
The duffer, average, scratch and pro golfer can determine the target line in his stance position or at least a line reasonably close to a true line for all practical purposes. Some golfers at stance have difficulty in accurately locating the putter face normal to the envisioned line. Most golfers have difficulty in maintaining the putter face at a right angle to the envisioned line during the entire stroke. But all golfers have difficulty in stroking the putter against the ball on a straight line.
The non-straight line stroke is the element resulting in greatest error. And the non-straight line stroke persists because the golfer has no way of measuring the straight line accuracy of his stroke.